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Author Topic: Delta Loop Questions  (Read 1800 times)
AD6KA
Member

Posts: 2232




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« on: December 30, 2012, 09:23:50 AM »

I am considering putting up a 20m Delta Loop
in my antenna farm. It will be supported by an
approx 40' mast that is set in cement on the ground
and also bolted to the eaves of
my house at the 20' level, just below the
roof line.   (I know that doesn't
leave room at the bottom for a perfect triangle
as the flat bottom section would hit the roof,
but I will do the best I can with what I have).
The Apex of this loop will hang from a 6 foot wide "tee"
and pulley I already have on the top of the mast.
There is an Alpha Delta DX-DD hanging from the
other side. I don't see much interaction happening,
but is it something to be concerned about?


I do not plan to use it on any other bands.
Ideally it will be optimized for 20m. But if it tunes
up and works OK on 10m with a particular feedline method,
I won't be averse to trying it.

I know that a delta loop fed from the bottom or top
is horizontally polarized and can be vertically polarized
by feeding it  from a corner at the base. I want to first
try it with the vertical polarization, so I will feed
it at the corner.


I would like to know the pros and cons of various
feedline methods:

1) Ladder Line to tuner.(Kinda problematic with my setup)
2) Ladder Line to a balun and short coax lead-in to tuner in shack.
(What kind of balun would be best here?)
3) Coax fed with 4:1 W2AU type balun at antenna.
I saw this setup while researching delta loops.
(Would a coax wound choke also be recommended here?)
4) 1/4 wave 75 ohm coax matching stub to loop,
attached to 50 ohm coax to shack?
5) Any other recommendations?

Thanks so much for letting me bend your ear!
Any and all advice is appreciated!
TNX ES 73, Ken AD6KA

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ZL1BBW
Member

Posts: 346




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« Reply #1 on: December 30, 2012, 10:19:45 AM »

For me, I would use ladderline to a 4:1 balun then a short piece of coax into shack, and a tuner if you have one. 
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
KJ6HZ
Member

Posts: 16




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« Reply #2 on: December 30, 2012, 11:59:38 AM »

Hi Ken,
Ladder line is great if you want to use it for multi-band, but for mono-band it would be much easier to just use a 4:1 balun (still a mis-match) or a quarter wave transmission line transformer made from 70 ohm coax.  The last is probably the easiest.  I don't know that I'd worry too much about a balun but you could always wind a few coils from the 70 ohm section if you prefer.   

Note that some instructions for vertically polarized loops recommend feeding 1/4 wavelength down the diagonal leg from the top.  This is close to the bottom corner but not quite the same.  I presume this method gets a more purely vertical polarization.  Ordinarily I wouldn't worry too much about it but it may help minimize coupling to the adjacent horizontal antenna in this case.

John KJ6HZ
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WB6BYU
Member

Posts: 13010




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« Reply #3 on: December 30, 2012, 01:25:15 PM »

Optimum vertical polarization is to feed the loop 1/4 wavelength down one of
the sloping sides, which isn't quite all the way to the corner.

The impedance will depend where you feed the loop and its dimensions.
Normally, with an equilateral loop (all sides the same length) it wouldn't make
any difference, but there is some variation due to interaction with the ground,
and if the loop is somewhat flattened to fit above the roof (so it is wider than
it is tall) that also affects the feedpoint impedance.

I modeled a 20m delta loop with the top at 40' and the bottom wire at 22',
which required about 28' for the bottom wire.  For vertical polarization (fed
about 4 1/2' up from the bottom of one of the sloping wires) the feedpoint
impedance is about 70 ohms, so direct coax feed with a 1 : 1 balun would
be the best choice (SWR = 1.4 : 1).  But when fed in the middle of the
bottom wire or at the top of the delta for horizontal polarization the feedpoint
impedance is around  170 ohms, and a 4 : 1 balun would be better
(SWR = 1.2 : 1).

If you use a 1 : 1 balun and 1/4 wave of 75 ohm coax it splits the difference
and the SWR is about 1.6 : 1 for vertical polarization and 1.5 : 1 for horizontal.
With this approach you could put pulleys in the 3 corners of the loop and rotate
the wire (and feedpoint) between the top and the bottom of one side, allowing
you to experiment with both options (as well as some in between).


But note that these impedances depend on the exact shape of the loop.  As it
gets shorter and wider it looks more like a folded dipole for horizontal polarization
(the impedance goes up) while the impedance goes down for vertical polarization.

For my model, a total vertical height of 14' for the loop (top at 40', bottom at 26')
gave a good match to 50 ohms for vertical polarization, though that will vary with
the soil conditions and coupling to wiring, etc. in the house.  It also doesn't include
any coupling between the loop and the feedline, which may be difficult to eliminate.
But it's a convenient place to start experimenting.
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AD6KA
Member

Posts: 2232




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« Reply #4 on: December 30, 2012, 02:36:43 PM »

Hey Guys:
Thanks much to Gavin, John, and Dale for your replies,
with special thanks to Dale for modeling my proposed antenna!
Quote
WB6BYU:
For vertical polarization (fed about 4 1/2' up from the bottom of one of the sloping wires) the feedpoint impedance is about 70 ohms, so direct coax feed with a 1 : 1 balun would be the best choice (SWR = 1.4 : 1).
Thanks, I didn't know this.
Quote
If you use a 1 : 1 balun and 1/4 wave of 75 ohm coax it splits the difference and the SWR is about 1.6 : 1 for vertical polarization and 1.5 : 1 for horizontal.
With this approach you could put pulleys in the 3 corners of the loop and rotate the wire (and feedpoint) between the top and the bottom of one side, allowing you to
experiment with both options (as well as some in between).
This sounds like an EXCELLENT approach as I
WILL be pulling the wire through the pulleys
trying to get everything lined up as best I can.
I do have a couple wide range tuners but I *plan* Cheesy
to get the SWR as low as possible. I am happy
to hear that the 75 ohm coax matching stub will
work well.  Getting ladder line into my shack,
while not impossible, would be difficult.
TNX AGN ES 73, Ken  AD6KA

Great info gentlemen, just what I needed
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W7KB
Member

Posts: 57




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« Reply #5 on: January 20, 2013, 08:58:43 PM »

If the impedance of a delta loop is about 100-120 ohms,just curious why a 2:1 balun instead of a 4:1 balun isn't recommended for use?...Dennis Reagin W7KB.
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N4CR
Member

Posts: 1650




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« Reply #6 on: January 20, 2013, 09:07:13 PM »

If the impedance of a delta loop is about 100-120 ohms,just curious why a 2:1 balun instead of a 4:1 balun isn't recommended for use?...Dennis Reagin W7KB.

It would be if you intend to hang the loop on the end of 50 ohm coax and stay on the antenna's tuned frequency.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB6BYU
Member

Posts: 13010




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« Reply #7 on: January 20, 2013, 10:12:08 PM »

Quote from: W7KB

If the impedance of a delta loop is about 100-120 ohms,just curious why a 2:1 balun instead of a 4:1 balun isn't recommended for use?



Making a proper 2 : 1 impedance ratio balun isn't as simple as it sounds, especially if
it is designed to operate over a wide frequency range.  Typically a balun will have an
integer turns ratio between two sides, and the impedance ratio is the square of the
turns ratio.  You can make a voltage balun using a tapped transformer, but that has
limitations, and getting a good current balun in other multiples is certainly not easy.

A 1 : 2 turns ratio gives 50 : 200 ohms, and is comparatively simple to make.  Also,
for good wideband balance, you'd like the coax shield to connect to the center tap
of the secondary winding, so the secondary should have an even number of turns.
A 2 : 3 turns ratio would match 112 ohms, but wouldn't allow you to ground the
center tap of the secondary easily.  A 3 : 4 ratio would match 89 ohms, but that
would traditionally require a septafiliar winding, and the coupling among the wires
in the bundle becomes more questionable as you get beyond 3.

That's why, for broadband use, the 1 : 1 and 4 : 1 baluns are the most practical
units.  Other ratios are possible, but performance tends to be less reliable.

Besides, for monoband use, the quarter wave transformer is much easier.  You can
wind it up into a choke if desired.

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